I would find out about what your eligibility for maternity leave is at your new job and what this means for not returning. You have to work for your current employer for 26 weeks prior to your qualifying week, which is the 15th week before your due date, so basically 41 weeks before your due date in order to qualify. It sounds like you will have done that since you aren't even pregnant yet. So that should be fine. I know in some jobs, if you claim SMP, you have to return for a period of time after your maternity leave to 'pay back' in a way your time off on maternity leave. You might just check what your employer's policies are on that. Also, do you plan to ever go back to work? And even if you say no now, realistically you might change your mind if circumstances change anyway. Would leaving on maternity and not returning as planned jeopardise getting a reference for another job later on? Those would be the two things I'd think about.
I wouldn't worry about the baby weight for the wedding. You'll probably have lost some, but you also probably won't care as much as you might have in the past. The only thing I would think about is just if you feel you need some time as a married couple first before you have children. I know I really enjoyed the time after we got married when it was just us and we didn't have anyone else to worry about and we could go have a nice honeymoon and relax (you won't get to do these things easily once you have a baby. I miss sitting down and reading and having a cocktail when we go away!). If you've been together a long time and you've lived together awhile already, it may not matter as much, but it's definitely something to think about. My husband and I lived on opposite sides of the planet for the 2 years prior to our wedding, so once we got married, it was so nice to just be together and get a house ready and relax and travel and just have time together the two of us. It would have been much more difficult with a baby and you never get those couple days back again once you become parents (we've had dinner out alone like maybe 10 times in 2.5 years). So it's just something to think about. I wouldn't worry about the baby weight, but I would just be mindful of if you're ready for it to be more than just two of you before your married, or if you want some more time just for yourselves, including celebrating your wedding.